I worked on a reggae festival 12 years ago where I fell asleep at the console and destroyed some subwoofers. I fell asleep because I just wasn’t prepared for such a long day and didn’t have anyone to take over for me. I’m still not completely clear on how the subs were destroyed, but I think it was a combination of improper limiter settings and over driving them on accident because I was mixing from a power valley and some issues with the crossover alignment.

To avoid repeating this mistake, I have dedicated this article to learning more about the repercussions of misalignment. Is subwoofer crossover alignment important? If so, why? 

Introduction

When it comes to live events, one of the most important things to get right is the bass. This drives the power and excitement that make it worthwhile for anyone to buy a ticket, fight traffic, and queue for an hour to see your show. On top of that, you’ve busted your ass to plan the sound system and set it up right. The last thing you want is to fight with the mix all night because the alignment is off.

What you’ll discover over the course of this article is that subwoofer crossover alignment is critical not only for an exciting show, but also to accurately reproduce the performance, protect the equipment and audience health, and keep from pissing off the neighbors.

What is Subwoofer Crossover Alignment?

Subwoofers are used to extend and support the operating range of a sound system in order to completely cover the entire human hearing range. Although many modern full-range speakers are technically capable of doing it all alone, subwoofers bring an important level of power and efficiency required by modern concert sound and even in many musicals and corporate events. Anecdotally, I’ve noticed that an audience may not notice when subs are missing, but they will surely notice if subs are there and then taken away.

The crossover frequency range where the main (aka full-range) and sub share joint custody must be time and phase aligned to to produce summation through the intended audience area. 

For more on this please see Do subwoofers need time alignment?

How important is subwoofer crossover alignment, really?

To obtain maximum performance from your system, it is important that all the different parts of the system are optimised to work together. In its simplest form, this is the process of ‘time alignment’. This should be considered as crucial to the successful deployment of the system as is the placement loudspeakers and their splay angles.

Coda Audio, Time Alignment Guide LINUS Control v2.3

Any time I post about this topic it seems to generate some controversy. Some people even seem offended that I focus on it as much as I do.

Every time I ask people about what they need help with, alignment, phase response, and subwoofers are always at the top of the list. I have chosen to embrace it and offer any help that I can. 🙂

We know sub alignment isn’t the most important thing on the list. Without power there’s no show. It’s also not the least important thing because your precision EQ isn’t worth a damn against the nulls of a comb filter.

I’m going to propose an easy way of thinking about this: dollars to decibels. $ = dB

Imagine that you have purchased a $9k sound system. If we agree that the acoustic crossover region between main and sub will affect about one octave out of the nine total octaves of the operating range, then the alignment is worth about $1k. It might not bankrupt us, but we also don’t want to leave $1k on the table. We’ll miss it if it’s gone.

At the end of the day it’s one of many things that need to get done, but it’s not worth obsessing over and shouldn’t be a time-consuming focus of our day.

It’s interesting to note that in the fundamentals training from L-Acoustics, System & Workflow, that the very first steps you learn to take with the smallest system setups are:

1. Load the correct DSP preset.
2. Main-sub alignment (geometrical).

Let’s dig into why this might be.

Sonic Performance

The biggest benefit to having a good sub/top alignment is having a freq response that doesn’t have any holes or peaks in the response. Or as some say “I can hear all of the notes the bass player is playing”.

Ivan Beaver, Danley Sound Labs

To understand this topic better I reached out to several audio engineers that I respect. In their responses the goal of sonic performance came up the most often. From a manufacturer’s perspective, I guessed they would care most about making sure it was loud and giving every customer the decibels they paid for, so I was happily surprised to read so many responses about minimum variance.

[With proper alignment there is] consistency in the mix for instruments that transition through the crossover frequency region. This typically includes cello, upright bass, and many acoustic instruments as well as synth sounds. With improper alignment, those instruments will change in volume, depending on what notes they’re playing.

Nick Malgieri, d&b audiotechnik

Other people call this a flat or equal amplitude response, meaning that you get out of it what you put in. This is the goal of a minimum variance sound system. It draws as little attention to itself as possible and delivers the same show to everyone in the audience. That way everyone in the audience gets to hear what the artist, presenter, and mix engineer intended.

Of course, a flat frequency response target is not necessarily the only or best choice. Many times the target is tilted and modified in response to reverb time, appropriate sonic image, and program content. Several popular loudspeaker manufacturers use a default target that is not flat.

Here is the default target in a blank SoundVision design showing +12dB of boost around 60Hz.

Beyond frequency response, sound system specifications include various other parameters that will be affected by sub alignment.

One of the biggest benefits to proper sub alignment is in achieving the design and calibration objectives including the targeted frequency response (tonal balance is the same everywhere in the audience), system headroom, and gain-to-feedback to fit the real-time demands of a production.

Vic Wagner, L-Acoustics

Efficiency

Large events can have high SPL requirements at long distances. Their program content may be highly dynamic and have a high crest factor. Combine this with the fact that the low end of the operating range requires the largest share of electric amplification energy and you need a system whose headroom is carefully controlled.

With so much power being pushed into the crossover region of main and sub we want our system to be calibrated to use that energy as efficiently as possible. The goal is maximum summation through the entire crossover region.

In this example plot taken from SubAligner, you can see that the sum in pink between the ULTRA-X40 and the 750-LFC match the the maximum possible target in black.

Poor time alignment and placement of subwoofers will create more destructive interference and less overall output from a given sound system.¹

Coverage

Overlap onto walls

The first rule of sound system design is to put sound where the people are and not where they are not. The ideal is a sound system design whose coverage shape perfectly matches the audience while avoiding the stage, walls, and neighbors.

One risk of misalignment is an incongruent spatial system response. It’s possible to design your system well, but then align it poorly and end up with unintended lobes into the targets you wanted to avoid. This is the risk I want you to be aware of if you choose to ignore alignment altogether. Just because you haven’t verified the alignment doesn’t mean it’s not aligned somewhere, you just don’t know where. Instead of being aligned in the audience, you may end up being aligned into the ceiling.

What’s wrong with being aligned into the ceiling or overlapping onto walls? Consider that Niels Adelman-Larsen wrote an entire book about the importance of the 125Hz octave band and controlling its reverb time. Two of the biggest factors affecting reverb time are number of sources and their directivity. If a misalignment is directing unwanted sound onto walls and open mics, the reverb time is going to go up.

It is a fact that the more sound the PA system shoots onto the walls and ceiling, the more the reverberation of the hall is evoked.

Adelman-Larsen, Niels. Rock and Pop Venues. Cham, Switzerland, Springer Nature, 2021.

One octave centered at 125Hz = 88-177Hz. Depending on the amount of haystacking, your subwoofer crossover alignment will play a greater or lesser role in this area.

For more on this, please see For the Best Concert Sound, Keep the LF Reverb Time Low and Why is the 125 Hz band so important?

Overlap onto the stage

One of the biggest risks of misalignment is “rogue” frequencies whose dispersion characteristics are louder in the direction of open mics. For indoor shows, it’s “rogue” frequencies whose dispersion characteristics are louder in the direction of reflective surfaces such as walls and ceilings. These indirect sounds cause muddiness, loss of impact, and sometimes cancelation in the audience or mix position. Now the engineer is trying to boost those missing frequencies (which won’t work) and can make microphone feedback even worse.

Nick Malgieri, d&b audiotechnik

This is a big one that I didn’t consider before hearing it from Nick. One of the biggest factors affecting gain-before-feedback is directivity. We carefully place and aim our speakers away from the stage, in part, to avoid feedback. 

If you’ve done some experimenting in modeling software or read The Complete Guide to to Measurement Microphone Placement for Subwoofer Alignment you know that as you adjust the phase alignment between main and sub, and therefore the location of their summation lobe, there is a mirror action happening behind the array, on the stage. Depending on the alignment, the location of this rear lobe may end up going directly into your open vocal mikes.

Here’s an example of an alignment into the ceiling and its mirror reponse in the rear.

Here’s the same design, this time aligned into the audience.

Well planned subwoofer placement and optimization will not only benefit the audience but the performers as well.¹

Cancellation at the mix position

Besides accidentally putting sound into open microphones and onto walls, which could screw up your show, there may be a bigger risk in actually damaging someone’s hearing. Imagine the classic case of a deep power valley created by an uncoupled sub array. Now imagine that you’re mixing from that valley. If you’re in a rush and unaware of what’s happening across the audience you may turn up the bass so much that it reaches unhealthy levels at the first row while you’re still barely hearing it at FOH. With ground based subwoofers you’re at a further disadvantage based on the extreme front to back ratio.

One of the biggest risks of misalignment is adversely affecting the auditory health of the audience and operating outside of the restrictions of noise abatement policy.

Vic Wagner, L-Acoustics

Here’s a YT video I made for a workshop in attempt to demonstrate this phenomenon.

A common objection

Perfect alignment is a myth because it can only be true at a single location.

This is true! But, it’s also not very helpful.

Alignment is always a compromise. Choosing an alignment position at one location means there will be misalignment at other locations. The perfect alignment (compromise) results in the least misalignment across the intended coverage area.

Tools like Merlijn van Veen’s Sub Align calculator can be used to discover the alignment position with the best shot at putting the majority of the audience into the coupling zone.

Tonality

While it is unreasonable to expect consistent sound levels across an entire venue (due to propagation loss) – at least without a complicated system – it isn’t out of the question to expect consistent tonality at all audience locations.2

Tonality is related to sonic performance and coverage. It makes me think about a recent outdoor concert where I made a big mistake with a sub arc. I was so focused on matching the sub array’s coverage shape with the audience that I didn’t consider connecting it with the coverage shape of the mains. The main’s coverage was wider than the sub’s. It sounded great in the middle, but as soon as I walked into the transition area where the sub coverage stopped, ooof, it was painful. 

A general ideal design goal is to match the audience shape with the main, sub, and the crossover region between them. You could be well matched with the first two, but then create abrupt tonality shifts by neglecting the third.

Equipment health

Let’s go back to our imaginary example of mixing from a power valley (in between the null of two summation lobes). The worst case scenario is that without monitoring system health you’ll continuously hit the limiters and fatigue electro-acoustical components. This could be further exacerbated with an under powered system where you start to push the console into clipping. This leads to damaged electronics or risk of fire as wiring heats up beyond its safety limit.

I see a lot of post-mortem log files from our amps reporting under-voltage or under-frequency reports on AC mains followed by generator shutdown or worse. The software protection algorithms associated with the factory presets are very good at preventing motor failure or overdrawing mains inlet current, but they don’t protect against product misuse or disappointed customers.

Vic Wagner, L-Acoustics

Impact

Misalignment will cause so-called time smearing; where the sound from different bands doesn’t arrive at the same time. This might be inaudible for continuous waves, but it will cause transient distortion.³

As sound engineers our job is to manage the relationships between many sources. One of the biggest challenges is to maintain alignment so that the combined impulse response arriving at the listener is not mixed up and stretched out. While it is unlikely that you’ll be able to perceive separate arrivals when main and sub are misaligned, there may be an overall lack of impact as compared with a fully aligned system. Furthermore, the two sources won’t sum correctly and the resulting ripple through the crossover region will sound like an EQ problem.

This is one of the risks you take if you use any single point alignment solution like listening to an inverted sine wave at the “crossover frequency”. While you may be phase aligned at that point, and therefore hear cancellation, you may be out of time by a whole cycle or more.

A case for ignoring alignment

When it comes to alignment, I just use my ears. At the end of the day, that’s what the audience is using.

One of my theories for why alignment can sometimes go wrong is that the ripple from misalignment might actually sound good, at least in one location. Imagine that you are standing at front of house listening to music and attempting to adjust the subwoofer delay time by ear. As you audition different delay settings you discover one that results in a cut in an area of resonance that was bothering and a boost at a nice point of impact for the kick drum. To your ears, you’ve made the right choice. Unfortunately, this may only be a local phenomenon, damaging the response for the rest of the audience, and it may have unintended system response consequences down the road when the program material isn’t exactly like your test track.

Conclusion

The importance of subwoofer crossover alignment is highlighted in this article, as it can be used to create an exciting show, faithfully represent the performance, protect the equipment and the audience, and prevent disruption of local noise limits.

It’s not something to obsess about, but it should be included in every system calibration workflow, big or small. Learn more about how to save time on sub alignment over at SubAligner.

References

¹AD. J.. Hill, MA. O.. Hawksford, AD. P.. Rosenthal, and G. Gand, “Subwoofer Positioning, Orientation, and Calibration for Large-Scale Sound Reinforcement,” Paper 7971, (2010 May.).

²AD. J.. Hill, “Live Sound Subwoofer System Performance Quantification,” Paper 9925, (2018 May.).

³Bävholm, Dan and Johny Grenander. “Delay alignment of top- and sub loudspeaker systems; a survey of new and old methods.” (2010).